md/bitmap: add blktrace event for writes to the bitmap

We trace wheneven bitmap_unplug() finds that it needs to write
to the bitmap, or when bitmap_daemon_work() find there is work
to do.

This makes it easier to correlate bitmap updates with data writes.

Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Shaohua Li <shli@fb.com>